1. Field of the Invention
The present invention is directed in general to television modulators. In one aspect, the present invention relates to a method and system for generating a carrier tone for an audio signal that is synchronized to an associated video signal in accordance with established standards for the broadcast of stereophonic cable and television signals in the United States and in other countries. In a further aspect, the present invention provides an integrated circuit system for generating an aural pilot subcarrier for use in digital BTSC stereo encoding and decoding.
2. Related Art
In 1984, the Federal Communications Commission (FCC) adopted a standard for the audio portion of television signals called Multichannel Television Sound (MTS) Transmission and Audio Processing Requirements for the BTSC System—OET-60, which permitted television programs to be broadcast and received with bichannel audio, e.g., stereophonic sound. Similar to the definition of stereo for FM radio broadcast, MTS defined a system for enhanced, stereo audio for television broadcast and reception. Also known as BTSC stereo encoding (after the Broadcast Television System Committee (BTSC) that defined it), the BTSC transmission methodology is built around the concept of companding, which means that certain aspects of the incoming signal are compressed during the encoding process. A complementary expansion of the signal is then applied during the decoding process.
The original monophonic television signals carried only a single channel of audio. Due to the configuration of the monophonic television signal and the need to maintain compatibility with existing television sets, the stereophonic information was necessarily located in a higher frequency region of the BTSC signal, making the stereophonic channel much noisier than the monophonic audio channel. This resulted in an inherently higher noise floor for the stereo signal than for the monophonic signal. The BTSC standard overcame this problem by defining an encoding system that provided additional signal processing for the stereophonic audio signal. Prior to broadcast of a BTSC signal by a television station, the audio portion of a television program is encoded in the manner prescribed by the BTSC standard, and upon reception of a BTSC signal, a receiver (e.g., a television set) then decodes the audio portion in a complementary manner. This complementary encoding and decoding ensures that the signal-to-noise ratio of the entire stereo audio signal is maintained at acceptable levels. FIG. 1 is a block diagram of the front end portion of an analog BTSC encoding system 100, as defined by the BTSC standard. Encoder 100 receives left and right channel audio input signals (indicated in FIG. 1 as “L” and “R”, respectively) and generates a conditioned sum signal (“L+R”) and an encoded difference signal (“L−R”). It will be appreciated that the processing of the sum signal (“L+R”) and difference signal (“L−R”) by the encoder 100 is substantially as described in the Background section of U.S. Pat. No. 5,796,842 which explains that the BTSC standard rigorously defines the desired operation of the encoding filters in terms of idealized analog filters.
To create the stereo signal, the BTSC standard also defines a composite stereophonic baseband signal (referred to hereinafter as the “composite signal”) that is used to generate the audio portion of a BTSC signal. The composite signal is generated using the conditioned sum signal (“L+R”), the encoded difference signal (“L−R”), and a tone signal, commonly referred to as the “pilot tone” or simply as the “pilot,” which is a sine wave at a frequency Fp, where Fp is equal to 15,734 Hz.
FIG. 2 is a graph of the spectrum of the composite signal. In FIG. 2, the spectral band 202 containing the content of the conditioned sum signal (or the “sum channel signal”) is indicated as “L+R.” The spectral sideband 204 containing the content of the frequency shifted encoded difference signal (or the “difference channel signal”) is each indicated as “L−R,” and the pilot tone 210 is indicated by the arrow at frequency Fp. In addition, the spectral sideband 206 containing the content of the frequency shifted encoded secondary audio program (or the “secondary audio channel”) is each indicated as “SAP,” and the spectral sideband 208 containing the content of the frequency shifted professional channel is each indicated as “Professional Channel.”
The encoded “L+R” and “L−R” signals are transmitted to the receiver, such as a stereo television set, where a stereo decoder uses both the “L+R” and “L−R” signals in a matrix that decodes and restores the original L and R audio program. For purposes of transmitting a BTSC encoded signal, a third signal, called the pilot subcarrier signal 210, is inserted between the main-channel signal 202 (L+R) and the stereo signal 204 (L−R), as illustrated in FIG. 2. According to the BTSC standard, the “pilot subcarrier shall be frequency locked to the horizontal scanning frequency of the transmitted video signal,” and may be used to indicate the presence of multiple sound channels or to process these sound channels at the receiver. The composite signal is generated by multiplying the encoded difference signal by a waveform that oscillates at twice the pilot frequency according to the cosine function cos(4π Fpt), where t is time, to generate an amplitude modulated, double-sideband, suppressed carrier signal and by then adding to this signal the conditioned sum signal and the pilot tone.
In the past, BTSC stereo encoders and decoders were implemented using analog circuits. Prior attempts to produce a sinusoidal pilot signal have used sinusoidal input tones or otherwise analog signal tracking techniques to have a sinusoidal pilot tone track the horizontal scanning frequency of the transmitted video signal. However, conventional analog BTSC encoders (such as described in U.S. Pat. No. 4,539,526) have been replaced by digital encoders because of the many benefits of digital technology. Prior attempts to implement the analog BTSC encoder 100 in digital form have failed to exactly match the performance of analog encoder 100. This difficulty arises from the fact that the BTSC standard defines all the critical components of idealized encoder 100 in terms of analog filter transfer functions, and prior digital encoders have not been able to provide digital filters that exactly match the requirements of the BTSC-specified analog filters. As a result, conventional digital BTSC encoders (such as those described in U.S. Pat. Nos. 5,796,842 and 6,118,879) have deviated from the theoretical ideal specified by the BTSC standard, and have attempted to compensate for this deviation by deliberately introducing a compensating phase or magnitude error in the encoding process. To the extent that conventional digital encoders have attempted to provide a digital tracking mechanism for the aural pilot subcarrier, conventional digital techniques have had only limited resolution due to sampling rate limitations, or have used complex filter structures that contribute to circuit size and cost. In addition, conventional digital phase and tracking loop approaches have used clock rates similar in frequency to the repetition rate of the input reference clock, and as a result have suffered from poor acquisition/tracking performance.
In addition to the complexity of the computational requirements for encoding the stereo signals, such as described above, the ever-increasing need for higher speed communications systems imposes additional performance requirements and resulting costs for BTSC encoding systems. In order to reduce costs, communications systems are increasingly implemented using Very Large Scale Integration (VLSI) techniques. The level of integration of communications systems is constantly increasing to take advantage of advances in integrated circuit manufacturing technology and the resulting cost reductions. This means that communications systems of higher and higher complexity are being implemented in a smaller and smaller number of integrated circuits. For reasons of cost and density of integration, the preferred technology is CMOS. To this end, digital signal processing (“DSP”) techniques generally allow higher levels of complexity and easier scaling to finer geometry technologies than analog techniques, as well as superior testability and manufacturability.
There is a need to provide a digital encoding system for processing stereophonic audio signals in compliance with the BTSC standard that provides accurately encoded audio signals. Conventionally known systems have used overly complex circuits to generate pilot signals with insufficient frequency resolution. Further, the nature of existing analog BTSC encoders has made them inconvenient to use with digital equipment such as digital playback devices. A digital BTSC encoder could accept the digital audio signals directly and could therefore be more easily integrated with other digital equipment. Therefore, there is a need for a better system that is capable of performing the above functions and overcoming these difficulties without increasing circuit area and operational power. Further limitations and disadvantages of conventional systems will become apparent to one of skill in the art after reviewing the remainder of the present application with reference to the drawings and detailed description which follow.